Single Wall Carbon nanotubes (SWNTs) can be realized as graphite sheets that have been rolled into seamless cylinders. Ever since Carbon nanotubes (CNTs) discovery by Iijima in 1991, they have been treated as the most promising nanostructured materials. S. Iijima, “Helical microtubules of graphitic carbon”, Nature, vol. 354, pp. 56-58, November 1991. Carbon nanotubes exhibit both semiconducting and metallic behavior depending on their chirality. J. W. G. Wildoer, L. C. Venema, A. G. Rinzler, R. E. Smalley, and C. Dekker, “Electronically structure of atomically resolved carbon nanotubes”, Nature, vol. 391, pp. 59-61, January 1998. This special property of nanotubes makes them the ideal choice for interconnects and also as active devices of nanoelectronics. CNTs have been used as chemical sensors for the detection of hazardous gasses such as NH3 and NO2. J. Kong, N. R. Franklin, C. Zhou, M. G. Chapline, S. Peng, K. Cho, and H. Dai, “Nanotube molecular wires as chemical sensors”, Science, vol. 287, pp. 622-625, January 2000. The application of these quantum wires as biological sensors is a new facet which might find significant applications in the life sciences field and it has been recently demonstrated that individual semiconducting single wall carbon nanotubes can be used for the detection of glucose oxidase. R. J. Chen, H. C. Choi, S. Bangsaruntip, E. Yenilmez, X. Tang, Q. Wang, Y. Chang, and H. Dai, “An investigation of the mechanisms of electronic sensing of protein adsorption on carbon nanotube devices”, Journal of American Chemical Society, vol. 126, pp. 1563-1568, January 2004. K. Besteman, J. Lee, F. G. M. Wiertz, H. A. Heering, and C. Dekker, “Enzyme-coated carbon nanotubes as single-molecule biosensors” Nanoletters, vol. 3, pp. 727-730, April 2003.
Recently acoustic wave sensors have been used for many applications in detecting chemical components in liquid media. By using the so-called chemical interfaces, they can be implemented for determining the concentration of a highly specific target compound in a liquid environment. The chemical interface selectively adsorbs materials in the solvent to the surface of the sensing area. Due to the change in the mass, the perturbation in the physical and chemical properties of the surface changes the phase and amplitude of the acoustic and electromagnetic fields on the surface. These changes can be monitored as the related change of mass.
Acoustic wave based sensors include those based on devices such as the Thickness Shear Mode (TSM), Surface Acoustic Wave (SAW), the Shear Horizontal Surface Acoustic Wave (SH-SAW), the Shear Horizontal Acoustic Plate Mode (SH-APM), and the Flexural Plate Wave (FPW). In a liquid environment, longitudinal bulk modes and Rayleigh waves cannot be used due to strong radiation losses into the liquid. Therefore, acoustic shear wave modes, which do not couple elastically to the liquid, are utilized; hence devices such as TSM, SH-SAW, Love modes, SH-APM and FPW are proper candidates for the development of devices to detect biomolecules in complex mixtures such as those represented by serum samples.
Since acoustic wave devices use piezoelectric materials for the excitation and the detection of acoustic waves, the nature of almost all of the parameters involved with sensor applications concerns either mechanical or electrical perturbations. An acoustic device is thus sensitive mainly to physical parameters, which may interact (perturb) with mechanical properties of the wave and/or its associated electrical field. For biological sensors, the binding of the antibodies and antigens on the substrate changes the mass of the membrane thus causing a drop in the wave velocity, which is correlated to the resonance frequency of the device.
Recent research in chemical sensing and microbiology has increased the quest for practical and inexpensive microfluidic devices. Different approaches for delivering samples through the microfluidic devices using micropumps have been investigated. However, most micropumps are not suitable for transporting fluid for this proposed microsystem due to performance dependency on temperature (thermal bubble pump and electrohydrodynamic pump), or concentration of ions in the sample (electroosmotic pump). Further, micropumps using valves or diffuser elements (electrostatically actuated pump and diffuser pump) are also not suitable as they present high impedance in the channel.
An acoustic micropump, such as the FPW micropump, has recently become known. The operating principle of this pump is based on the phenomenon of acoustic streaming, in which the fluid flows in the direction of the acoustic wave, eliminating valves, diffuser and dependency on temperature and ion concentrations. N. T. Nguyen, R. W. Doering, A. Lal, R. M. White, “Computational fluid dynamics modeling of flexural plate wave pumps”, Proceedings” IEEE Ultrasonics Symposium, Vol. 1, (1998) 431. N. T. Nguyen, X. Huang, T. K. Chuan, “MEMs-micropumps: a review”, Transactions of the ASME. Journal of Fluids Engineering, Vol. 124, No. 2, (2002) 384. N. T. Nguyen, A. H. Meng, J. Blac, and R. M. White, “Integrated flow sensor for in situ measurement and control of acoustic streaming in flexural plate wave micropumps, Sensors and Actuators A: Physical, Vol. 79, No. 2, (2000) 115.
Bradley et al have demonstrated the use of FPW micropump to produce a unidirectional flow with a velocity of about 150 μm/s. C. E. Bradley, J. M. Bustillo, R. M. White, “Flow measurements in a micromachined flow system with integrated acoustic pumping”, Proceedings: IEEE Ultrasonics Symposium, Vol. 1, (1995) 505. A conventional FPW transducer launches waves that add constructively in both the forward and the backward directions, thus giving bi-directional waves.